Electrically conductive elastomeric composite



1966 s. l. PATCHEN 3,281,624

ELECTRICALLY CONDUCTIVE ELASTOMERIG COMPOSITE Filed July 22, 1965 IN TOR STEPHEN I. TCHEN ATTOENEYJE United States Patent C) 3,281,624 ELECTRICALLY CONDUCTIVE ELASTQMERIC COMPOSITE Stephen I. Patchen, 4218 W. 63rd St., Cleveland, Ohio Filed July 22, 1963, Ser. No. 296,712 Claims. (Cl. 317-2) This invention relates to an electrically conductive elastomeric composition, and more particularly to electrically conductive elastic devices adapted to be worn, e.g. gloves, face masks, and electrically conductive elastic overshoes which may be worn over .the ordinary nonconductive type of shoe. These compositions may also be used to fabricate hoses, especially anesthetic conduits. For convenience, the invention will be described and illustrated with reference to electrically conductive elastic shoe covers, it being understood that composite structures hereof are useful in other applications, such as those mentioned.

During surgery, doctors and nurses are often required to remove and exchange their ordinary and non-conductive street shoes for conductive footwear before entering the operating room, or to slip over their street shoes bulky uncomfortable devices including body-grounding means. In like manner, other who work in the presence of explosive vapors, gases, dust and the like must also wear special conductive footwear or devices. A primary purpose of such shoes is to ground and dissipate static electric-ity which under circumstances usually attendant these occupations, is readily accumulated and stored by the human body. Electrostatic discharge from the body and articles of wearing apparel is [often-times sufficient to cause explosions of gases and dust, particularly explosions of high-1y volatile and inflammable anesthetic compositions.

The prior art devices have been largely characterized by inflexibility, bulkiness, discomfort to the wearer, and generally quite high cost.

The electrically conductive elastic shoe covers, embodying novel composites hereof, are also especially adapted to guard against what is termed cross contamination re sulting from bacteria or viruses on shoes. In hospital nurseries and communicable disease wards, cross contamination is a serious problem due to the transfer of infectious agents on shoes. The electrically conductive elastic shoes hereof are easily removed and may be repeatedly submitted to sterilizing procedures to mitigate this problem.

It is now been found that a highly effective electrically conductive elastic overshoe adapted to be worn over the wearers regular footwear may be inexpensively produced and adapted to a variety of footwear sizes utilizing an acetylene black filled elastomeric composition.

Acetylene black in the very finely divided form is peculiarly adapted for use in the production of composite electrically conductive elastic footwear, for the reason that this form of carbon maintains its conductivity under elastic stretching where other carbonaceous filler materials are either unable to form a conductive substance with a polymerized elastomer, or lose such conductivity as they may possess upon submission to elongation.

While the mechanism by which finely divided acetylene black maintains its conductivity in the relaxed state as well as in the stretched condition is uncertain, it is be lieved that very finely divided acetylene black having a particle size averaging from about 10 to 50 millimicrons in diameter has the property of agglomenating into nonlinear chains or loops of particles, held together by physical forces of molecular attraction and which in the composites of this invention are able to undergo stretching without breaking the particle-to-particle contact necessary to preserve electrical conductivity. Acetylene black particles also appear not to be encapsulated by the elastomer containing them in a manner which will destroy particle-to-particle contact as appears from studies to be the case with other finely divided carbon materials.

In the annexed drawings, there has been illustrated an electrically conductive elastic shoe cover embodying the present invention, and wherein:

FIG. 1 is a top elevation of a composite electrically conductive shoe cover of the invention showing the electrically conductive strip attached to the upper marginal edge of the heel portion thereof.

FIG. 2 is a side elevation of a composite electrically conductive overshoe showing the conductive sole and heel portions, and a non-conductive vamp portion in cohered relation.

FIG. 3 is a partial cross-sectional view of a composite electrically conductive elastic shoe cover shown in place on a conventional non-conductive shoe, and showing the conductive tail or body-contacting strip wrapped around the bare ankle of the wearer.

FIG. 4 is a partial cross-sectional view of a composite electrically conductive elastic shoe cover taken in the plane indicated by the line 44 of FIG. 1.

Briefly stated, the present invention is in an electrically conductive elastic composition including an electrically conductive elastic web, said conductive web containing finely divided aceytlene black uniformly dispersed therein to the extent of from about 6% to 50% by weight and said acetylene black having an average particle size in the range of from about 10 to about 50 millimicrons in diameter. Shoes, gloves, face masks, hoses and the like formed from composites of this nature are particularly useful in that they have good electrical conductivity and elasticity. 'lhese articles are capable of repeated stretching without substantial change in conductivity, and have good conductivity in the stretched as well as the relaxed condition.

The present invention is illustratively embodied in an improved electrically conductive elastic shoe cover adapted to be fitted over a variety of sizes of conventional noncondu-ctive shoes, and comprising in combination sole and heel portions of elas-tomeric materials containing intimately dispersed therethrough, acetlylene black in an amount ranging from about 6% to about 50% by weight, said acetylene black having an average particle size in the range of from about 10 to about 50 millimicrons, and an electrostatically conductive elastic strip or tail secured to the heel portion and adapted to be placed in electrostatic contacting relation with the wearers body.

Referring now more particularly to FIGS. 1 and 2, there is here shown an electrically conductive elastic shoe cover embodying the present invention having a nonconductive vamp portion 11, and a conductive sole portion 12 and a heel portion 13 in laminar intermolecularly cohered relation with the vamp portion 11. Heel portion 13 and sole portion 12 overlie and are in intermolecularly cohered relation with underlying non-conductive heel and sole portions as best shown in FIG. 4. Conductive strip or tail 14 is secured to the roller upper marginal edge 15 of the heel, and is of sufiicient length to be disposed inside the wearers normal non-conductive shoe to underlie the sole of the wearers foot between the sock and the shoe, or to be wrapped around the wearers bare ankle and tied in such position or tucked into the wearers sock. To strengthen the opening of the shoe cover, a peripheral bead 16 is conveniently provided, and when the shoe is formed by a dipping process, which is the preferred process, head 16 is conveniently formed by rolling back upon itself the upper marginal boundary of a portion of the vamp 11 and heel portion 13.

FIGS. 3 and 4 show partial cross-sectional view of the electrically conductive elastic shoes embodying the present invention and demonstrate the preferred cohered laminar structure in which the shoe is formed. Thus, the vamp portion 11 is formed by dipping a mold, such as a porcelain mold, into a latex formulation to a desired depth. Simultaneously there are formed non-conductive sole portion 11a and heel portion 12a. Sole portion 12 and heel portion 13 are formed in superimposed laminar cohered relation with vamp portion 11, sole portion 11a, and heel portion 12 by redipping the same non-conductive latex coated mold in an acetylene black filled latex composition of the same type as used to form the non-conductive portion. The extent of dipping to form the sole portion 12 is less than the extent of dipping to form the vamp portion 11, and superimposed cohered laminar heel portion 13 is formed by rotating the mold and immersing it to a depth sufficient to form the conductive heel portion as illustrated best in FIG. 2.

Preferably, although optionally, the shoe may be provided with a third cohered layer on the bottom of the heel portion 13 and on the bottom of the sole portion 12 which is a slip resistant layer 17 conveniently formed of the same acetylene black filled elastomeric composition of the conductive portions 12 and 13, but additionally including a relatively small percentage, e.g. about by weight of cotton linters, asbestos linters, or other non-skid material e.g. finely divided silica. Instead of an additive non-skid material, the shoe may be provided with a non-skid surface e.g. a roughened surface. As indicated, the application of such non-skid means is optional.

The preferred mode of making the electrically conductive composites of the present invention is by dip molding into a latex composition. The mold itself may be formed of porcelain, aluminum, heat resistant plastic, or even a wooden mold. Thus, tubular molds, glove molds, mask molds, etc, may be so utilized. The process will be illustrated by an elastic overshoe mold. The mold is made to the minimum of a given shoe size range, for example shoe sizes 8 to 12. A mold for this range will be sized to a shoe size 8, and the product which is obtained therefrom can be stretched to a size 12. Reference may be had to Patent No. 2,789,933, dated Apr. 23, 1957 for a suitable process of manufacture adapted to be used for manufacture of the present electrically conductive overshoes.

The first stage in the process is to dip the mold into a conventional ionizing coagulating agent e.g. a salt or acid. The molds are dipped warm, that is at a temperature of about 100 F. to 120 F. Following this, the molds are dipped either manually or automatically into a non-conductive elastomeric material (or latex) for the formation of the vamp 11 and inner sole 11a and heel 12a portions, as above indicated.

Following the dip into the non-conducting latex composition, the mold is then partially dipped into a tank containing a dispersion of acetylene black loaded latex, or a dispersion of acetylene black in a solution of an elastomer, the second dipping step being performed before substantial polymerization of the first layer occurs. Spontaneous curing times for latex compositions are well known and vary from a few minutes to several days. The conductive latex coats the latex covered mold to the extent indicated in FIG. 2 for the formation of the sole portion 12, including a welt portion 20. The mold is then rotated about a transverse axis to dip the heel portion and thus carry the conductive layer up to the top marginal edge of the heel portion of the non-conduc tive undercoating. 'In dipping the heel portion, it is desirable to apply a slight excess of the conductive material of the heel portion of the mold above the upper edge of the non-conductive coating. This insures that when the conductive strip 14 is rolled into the bead 16 at 15, it is not insulated from electrical contact 'with the heel portion bythe non-conductive layer. Finally, the coated mold may be touched to the surface of a flocked conductive latex solution to provide a conductive anti-slip surface onthe sole and heel, as indicated in FIGS. 3 and 4. Alternatively, this coating may be spray applied.

Conductive strips are made of the same basic acetylene black filled latex material, they being desirably molded on a continuous belt. The coating is doctored onto the belt and spontaneously cured by heat applied in an oven, or by electrical means. A wide sheet is separated from the belt and slit to provide the conductive strips or tails 14 eight inches to fourteen inches long and about 4; inch wide. The conductive strips are generally from 0.002 to 0.014 inch thick, e.g. 0.006 to 0.008 inch thick.

The strips are applied to the top of the heel portion and rolled into the bead 16 at the point 15 to secure the strip and establish a conductive junction at the back of the heel.

To cure the shoe cover, the mold is passed into an oven where it is retained for a period of about 40 to minutes at the curing temperature for the particular latex formulation. I

The molded shoe is then stripped from the form, washed and powdered for packing. In order to apply a finished surface to the shoe cover and improve its appearance, it may be dipped into a conventional finishing solution.

The shoes are then inspected and packed.

The present invention is not limited by the particular elastomeric material which may be used to form the electrically conductive composites hereof or the molding technique used. Thus, slush casting, or rotational casting may also be used. Accordingly, there may be used solutions of elastomeric materials in suitable solvents adapted for spray or dip application, or dispersions of such elastomeric materials in an aqueous medium, e.g. an elastomeric latex. Natural and synthetic rubber latices are preferred.

Examples of rubbers which can be used in making the elastic conductive composites hereof are any vulcanizable rubber including natural rubber, particularly Hevea rubber, rubbery copolymers of butadiene and styrene (so called GRS), polychloroprene (neoprene), rubbery copolymers of butadiene and acrylonitrile (so called Buna- N), rubbery copolymers of butylene and butadiene or isoprene (butyl), vinyl chloride polymers andcopolymers, etc.

Where solutions of elastomers are employed, the solvent may be such as toluene, xylene, benzene, methyl ethyl ketone, carbon tetrachloride, solvent. mixtures, e. g. butanol-toluene. When solutions are used, the solids content varies from about 3% to about 25%, the lower percentages being more satisfactory for spray application and the higher percentages more suited for dipping.

While milled rubber compositions adapted for molding may be used into which the acetylene 'black is introduced as a dry component, these have been found not as satisfactory in respect of conductivity for similar loadings as the latex compositions which are preferred.

With the exception of the utilization of the aforesaid concentrations of finely divided acetylene black, the latex or solutions of the el-astomer are conventional and the dipping or forming procedures are also conventional. Also, the non-conductive portions of the composite may be formed of one elastomeric composition and the conductive portion formed of the same or a different elastomeric composition. Thus, the non-conductive layer may be formed of one elastomeric composition and the :coneluding natural rubber as the elastomer and the conductive layer formed by dipping the uncured natural rubber latex coated mold into acetylene black filled (30% by weight of dry solids) butadiene-styrene latex (60% Solids) composition.

The adhesion between the layers is such that the respective layers of the composites hereof are virtually inseparable. Because the adhesion is secured without the interposition of any separate adhesive agent, it is believed that the bond between the two layers is predominantly chemical, and to describe this remarkable adhesion, the term intermolecularly cohered is used herein.

A typcal formulation for the conductive layer is as follows, the quantities indicated being parts by weight on the dry basis:

EXAMPLE I Parts Natural rubber (latex 40%) 100 Potassium hydroxide 0.5 Ammonium caseinate 0.5 Zinc oxide 0.25 Sulphur 0.75 Styrenated phenol (mono-, di-, tri-mixed) 1.0 Mercapto benzothiazole 1.5 Acetylene black (No. avg. 20 millimicrons) 30.0

The non-conductive layer may conveniently be formed from the same composition omitting the acetylene black component.

The foregoing composition is utilized as an aqueous dispersion at a concentration in the range of about 20% to about 60% solids. Conventional synthetic latex compositions of the butadiene-styrene, butadiene acrylonit-rile, neoprene, and curable formulations including such latices, all of which are well known, may be substituted for the natural latex in the above formulation and used in accordance with this invention.

The dispersion is further characterized by the inclusion therein of acetylene black having a particle size in the range of from to about 50 millimicrons in diameter, and included in the latex formulation to be the extent of from about 6% to 50% by weight of the entire composition on the dry basis. More satisfactory results are secured when the extent of filling with acetylene black is from about 18% to about 40% by Weight. Other ingredients may be included in amounts which do not adversely affect the composition for its intended purposes, such as pigments, fillers, dyes, antioxidizing agents, etc.

Generally, the acetylene black is introduced as an aqueous dispersion containing about 9% to 30% solids and formed from 100 parts of dry acetylene black, 2 parts of triethanolamine, 5 parts of a water soluble alkyl aryl sulphonate (sodium salt) and 5 parts of dimethylamine shellac soap. Any suitable aqueous dispersion of the acetylene black may be used so long as the dispersion acids will not adversely affect the latex composition, for example, by prematurely coagulating it.

The cured elastic acetylene black filled electrically conductive portions of the composites hereof have very low resistivities adequate to dissipate electrostatic charges at the same rate as the charges are generated, without the voltage at the point of generation rising sufliciently to cause an incendiary discharge, and therefore particularly adapted for use in an operating room where inflammable and/or explosive materials are present. At comparable loadings ordinary channel black and graphite give resistivities greatly beyond those which may be tolerated in operating rooms with safety. Surprisingly, among finely divided acetylene black, channel black, lamp black, and graphite dispersed in a given latex, at similar loadings and as cured thin films all but acetylene black demonstrate resistivities far too high for the conductivity necessary to insure safety in an operating room, for example. On stretching, channel blacks, lamp blacks and graphite, show ever poorer conductivity, or higher resistivity. Acetylene black gains in resistivity, 'but on stretching as much as twice normal length still possesses a resistivity low enough to meet hospital requirements, and a resilience more than enough to span 4 shoe size units. Thus, it is believed that acetylene black having the very small par- 6 ticle size mentioned above is unique in this respect and functions in a manner not common with other forms of carbon.

Other latex compositions useful in accordance herewith include the following:

EXAMPLE II Parts Butadiene-styrene (latex 40%) Sulphur 2.0 Octylated diphenylamine 1.5 Benzothiazyl disulphide 1.5 Acetylene black (20-50' millimicrons) 8 EXAMPLE III Parts Butadiene-acrylonitrile (latex 50%) 100 Sulphur 2.0 Styrenated phenol (mono-, di-, and tri-mixed) 1.0 Benzothiazyl disulphide 1.5 Zinc oxide 0.25 Acetylene black (20-50 millimicrons) 30.0

EXAMPLE IV Parts Poly (chloroprene) (latex 60%) 100 Zinc oxide 2.5 Magnesium oxide 2.0 Ethylene thiourea 1.0 Hydroquino-ne mono'benzyl ether 2.0 Acetylene black (2050 millimicrons) 20.0

The foregoing illustrative formulations may be utilized without the acetylene black component to provide the nonconductive layer as :above described. The corresponding acetylene black containing formulations are used to provide the conductive layer. When formed in the manner above exemplified, the composites provide useful structures for shoes, gloves, hoses, sheets, breather bags, inhalation cones, etc. It will be observed that the composites are so adapted and manufactured that different properties may be imparted to each element of the composite as may be required by the particular end use. This color may be imparted to the non-conductive layer to aid, for example, in identification of material carried by an electrically conductive hose formed from the composite; identification of shoe size range; etc. In each embodiment of the electrically conductive composite, one surface is electrically conductive and the other is nonconductive; these different properties being provided in a laminar composite structure in which the laminae are cohesively inseparab-ly united and no adhesive is used.

A further and particular advantage of the complete structures of the present invention is that the interface between the non-conductive layer and the conductive layer has been formed in the relaxed condition and in such a way as to provide an inseparable intermolecularly cohered laminar structure.

This structure is particularly valuable in that it has been found that the composites of the present invention are capable of withstanding repeated cycles of stretching and relaxation without substantial change in electrical conductivity. With simple films of carbon black loaded elastomeric compositions, repeated stretching and relaxation causes a loss of electrical conductivity. It has been suggested by others that this loss of conductivity results from breaking the chain like structures of carbon-to-carbon conductors within the body of the elastomer. In the present composites, however, the non-conducting intermolecularly cohered laminar portion is believed to aid in the preservation of the essential particle-to-particle contact in the acetylene black chains so that electrical conductivity is preserved through such flexures to a much higher degree than heretofore obtainable.

Other modes of applying the principle of this invention may be employed instead of those specifically set forth above, changes being made as regards the details herein disclosed, provided the elements set forth in any of the following claims, or the equivalent of such be employed. I

It is, therefore, particularly pointed out and distinctly claimed as the invention:

1. An electrically conductive elastic composite comprising a layer of a non-conductive elastomeric composition formed. from a latex composition and an elastic superimposed electricallyconductive layer intermolecularly cohered to said non-conductive layer, said conductive layer being an elastic elastomeric composition formed from substantially the same latex composition and addi-' tionally having disperse-d therein finely divided acetylene black in an amount ranging from about 6% to about.50%

by weight, said acetylene black having an average particle size in the range of from to about 50 millimicrons in' diameter.

2. An electrically conductive composite in accordance with claim 1 in which the non-conductive elastomeric composition and the conductive layer includes as the elastomer natural rubber.

3. An electrically conductive composite in accordance with claim 1 in which the non-conductive elastomeric composition and the conductive layer includes as the elastomer a synthetic rubber.

4. An electrically conductive composite in accordance with claim 1 in which the elastomer is a copolymer of butadiene and styrene.

5. An electrically conductive elastic shoe cover adapted to be worn over a variety of sizes of non-conductive shoes, comprising a unitary non-conductive elastic vamp portion, an elastic sole portion, and an elastic heel portion, a cohered electrically conductive sole portion overlying said nonc-onductive sole and a portion of said vamp portion, said non-conductive portions being formed from a first latex composition, a cohered electrically conductive heel portion conductively joined to said sole portion and overlying said non-conductive heel portion, an electrically conductive elastic strip portion conductively joined to said electrically conductive heel portion, each of said electrically conductive portions being formed from a latex of an elastomer of substantial-1y the same composition as said first latex composition additionally containing finely divided acetylene black dispersed therein to the'extent of from about 6% to about 50% by weight, said acetylene black having an average particle size in the range of from about 10 to about 50 millimicrons in diameter, said strip portion being of sufiicient length to be brought into conductive relation with the wearers body.

6. An electrically conductive elastic shoe cover in accordance with claim 5 in which the elastomer is natural rubber.

7. An electrically conductive elastic shoe cover in accordance with claim 5 in which the elastomer is synthetic rubber.

8. An electrically conductive elastic shoe cover adapted to be worn over a variety of sizes of non-conductive shoes,-comprising a unitary non-conductive elastic upper shoe portion having a foot receiving opening therein, a non-conductive elastic sole portion and a non-conductive elastic heel portion, said non-conductive portions being formed from a first latex composition, a cohered electrically conductive sole portion overlying said non-conductivesole portion and a portion of said upper shoe portion to form a welt portion, a cohered electrically conductive heel portion overlying said non-conductiveheel portion, an electrically conductive elastic strip portion conductively joined to said electrically conductive heel portion, a reversely rolled peripheral head portion surrounding and defining the opening into said shoe cover, said strip portion being secured at said head portion, each of said electrically conductive portions being formed from a latex of an elastomer having substantially the same composition as said first latex composition and additionally containing finely divided acetylene black dispersed therein to the extent of from about 6% to about 50% by weight, said acetylene black having an average particle size in the range of from about 10 to about 50 millimicrons in diameter, said strip portion being of sufiicient. length to be brought into conductive relation with the Wearers body.

9. An electrically conductive shoe cover in accordance with claim 8 in which the elastomer contains from 18% to 40% by weight of said acetylene black.

' 10. An electrically conductive shoe cover in accordance with claim 8 having on the sole portion an overlying cohered layer of conductive slip-resistant elastomeric material.

References Cited by the Examiner UNITED STATES PATENTS 2,302,003 11/ 1942 Cadwell et al 252-5 11 X 2,341,360 2/1944 Bulgin 317-2 X 2,407,189 9/1946 Taber et al 317-2 2,789,933 4/1957 Bargmyer 154-46 2,818,534 12/1957 Horne 317-2 3,013,184 12/1961 Adams 317-2 MILTON o. HlRS HFIELD, Primary Examiner. LEE T. HIX, Examiner. J. A. SILVERMAN, Assistant Examiner. 

5. AN ELECTRICALLY CONDUCTIVE ELASTIC SHOE COVER ADAPTED TO BE WORN OVER A VARIETY OF SIZES OF NON-CONDUCTIVE SHOES, COMPRISING A UNITARY NON-CONDUCTIVE ELASTIC VAMP PORTION, AND ELASTIC SOLE PORTION, AND AN ELASTIC HEEL PORTION, A COHERED ELECTRICALLY CONDUCTIVE SOLE PORTION OVERLYING SAID NONCONDUCTIVE SOLE AND A PORTION OF SAID VAMP 